The purpose of this proposal is to elucidate the amount of morphological variation within and among natural populations of Drosophila melanogaster which is actually due to genetic variation, to understand the selective significance of spatial and temporal variation of morphology of this species in nature, and to relate this variation to mi-gration behavior. The first objective will be accomplished by measuring the heritabilities of two traits within a population in nature, and to relate these to heritabilities determined in the traditional way in the laboratory. In addition, heritabilities of traits among populations will be determined by capturing flies along geographical and altitudinal gradients, measuring two characters on flies from nature, and relating these to the measurements on their laboratory-reared progeny. These studies will enable us to determine how much morphological variation seen in nature is due to genetic variation, how much to environmental influences, and the way in which genes and environment interact in nature. In addition, morphological patterns in traits which seem due to selective differences among populations will be studied in the laboratory to determine the nature of selection acting on the characters. Possible selective differences among natural populations will also be examined by studying geographic variation of important life history characters in D. melanogaster and their relation to temperature. Finally, the significance of geographic variation in both d. melanogaster and D. pseudoobscura will be studied indirectly by determining the ability of individuals to migrate and exchange genes over substantial distances in nature. The amount of such migration will indicate whether differences among populations in other traits such as allozymes may be ascribed to natural selection, or may be due to random genetic drift. The work has implications for human evolution and animal breeding since it forms a model system for questions about the inference of genotype from phenotype in nature, and about norms of reaction of genotypes. Such matters arise frequently in animal breeding studies and discussions of differences between human subgroups. In addition, the interaction of migration, drift, and selection studied here is of relevance to human subgroups. In addition, the interaction of migration, drift, and selection studied here is of relevance to human populations, which have become genetically differentiated for unknown reasons in the face of unknown amounts of migration.